——_______ go and help Huihui, a little girl who is badly ill in Zhaoyuan, Shandong?
——That's a good idea.
A:What about
B:Why don't
C:Can you
D:Why don't you
某同学用如图甲所示的装置通过研究重锤的落体运动来验证机械能守恒定律.已知重力加速度为g.
上古时代,人们住在野外的山洞里,后来最早学会建造房屋,开始过定居生活的是( )